A number of dyes for spectral sensitization (spectral sensitizing dyes) which are used in an electrophotographic light-sensitive layer containing a photoconductive material and a resin binder system are known. These spectral sensitizing dyes must fulfill various requirements. Particularly important properties among others include good adsorption onto the photoconductive material, high sensitizing efficiency, and a minimum necessary resistance of the electrophotographic light-sensitive layer in the dark. Examples of dyes satisfying the above requirements are described in U.S. Pat. Nos. 3,052,540, 3,110,591, 3,125,447, 3,128,179, 3,132,942, 3,241,959 and 3,121,008, and British Patent No. 1,093,823.
Spectral sensitizing dyes for sensitization in the range from red light to infrared are described in U.S. Pat. Nos. 3,619,154 and 3,682,630. These dyes, however, have the serious disadvantage for practical use that they are generally easily decomposed, during storage or during the process for preparing an electrophotographic light-sensitive layer containing such dyes or its storage, thereby reducing performance. In this regard, Harazaki et al. describe in Kogyo Kagaku Zasshi (Journal of Industrial Chemistry), Vol. 66, No. 2, p. 26 (1963) that sensitizing dyes for sensitization from red light to infrared are more unstable than those for short wavelength light (visible light).
In recent years, with development of low output semiconductor lasers, light-sensitive materials which are of high sensitization for long wavelength light of more than 700 nm have been extensively investigated. Cyanine dyes for spectral sensitization using zinc oxide as a photoconductive substance are described in Japanese Patent Application (OPI) Nos. 58554/83, 42055/83, 59453/83, etc. (the term "OPI" as used herein means a "published unexamined Japanese patent application").
However, the sensitivity of these cyanine dyes does not include the wavelengths from near infrared to infrared light, and the stability of the light-sensitive material is not sufficiently high, preventing the attainment of satisfactory high sensitivity.
Dyes which exhibit high sensitization efficiency to long wavelength light are earnestly desired, but dyes which satisfy this requirement are still not available.